The invention relates to an apparatus and a method therefor for detecting anomalous discharges accompanying a plasma discharge generated by a high-frequency voltage across a pair of electrodes supplied by a dc power source or a high-frequency power source. The invention also relates to an acoustic emission (AE ) sensor hold case.
Today""s semiconductor manufacturing industries widely use plasma discharge processing in chemical vapor deposition (CVD), ashing, etching, and sputtering of semiconductor substrates, as well as in surface processing thereof. Anomalous plasma discharges take place in a plasma processing equipment, which cause for example creation of dusts, damaging and/or contamination of semiconductor substrates, and electric breakdown of the electronic components on the substrates. In order to deal with these problems associated with anomalous discharges, accurate detection of occurrence and accurate determination of the location of an anomalous plasma discharge is necessary. There have been made extensive researches for a method to detect such anomalous discharges, which include detection of a change in the intensity of light emitted by the plasma, a change in the voltage and/or the current through a power supply, a change in plasma impedance, and a change of harmonic across the electrodes.
However, when monitoring the fluctuation in a plasma radiation for an anomalous discharge in a plasma processing equipment, relevant portions generating the plasma are covered with deposit shields if the frequency of the plasma is high. Therefore, it is necessary to modify many of the mounting equipment for the optical fibers. On the other hand, if the optical fiber is successfully mounted on the apparatus, characteristics of the plasma can be altered by the optical fiber. Process chambers having a viewing window also have a drawback in that accurate determination of the location of an anomalous discharge is difficult, since the entire plasma region cannot be observed through the viewing port.
A method of detecting an anomalous discharge through monitoring changes in the supply voltage of an RF power supply and/or the current though it, and in the plasma impedance, fails complete detection of the anomalous discharge. Furthermore, in this method, complete locations of the anomalous discharge cannot be known, since anomalous discharges can take place outside the electrodes. Although generation of an anomalous discharge can be attained by detecting changes in the harmonics modes of the plasma, the method cannot determine the positions of the discharges. Thus, presently there is no method known which is capable of both detecting anomalous discharges and determining the locations thereof.
In addition, there is a need for a method and means therefor for allowing easy mounting and dismounting of ultrasonic detection means (e.g. AE sensors) during maintenance, and preventing destruction and deterioration of the AE sensors caused by the stresses during mounting/dismounting of the sensors, thereby attaining a high precision ultrasonic detection of anomalous discharges by the AE sensors.
In search of adequate physical parameters which can be of good indication of an anomalous plasma discharge in a plasma discharge processing equipment, the inventors found a fact that ultrasonic waves accompany such anomalous plasma discharges and that they can be used as an indication of anomalous plasma discharges.
It is therefore an object of the invention to provide a method and an apparatus therefor for detecting anomalous plasma discharges in a plasma processing equipment by detecting an ultrasonic wave emitting from an anomalous discharge by means of ultrasonic detectors.
It is another object of the invention to provide a method and an apparatus therefor for accurately determining the position (hereinafter also referred to as source point) of an anomalous discharge in a plasma generating apparatus by means of multiple ultrasonic detectors placed at predetermined positions in the apparatus.
It is a further object of the invention to provide an apparatus which can estimate the level of the anomalous discharge by measuring the energy of an accompanying ultrasonic wave; display the location of the anomalous discharge detected; and generate an alarm if the level of the anomalous discharges exceeds a predetermined threshold level.
It is a still further object of the invention to provide convenient means for installing ultrasonic detectors on a plasma processing equipment.
To the accomplishment of the foregoing objects, there is provided an anomalous plasma discharge detection apparatus in accordance with one aspect of the invention, the apparatus comprising:
ultrasonic detection means, in the form of acoustic emission (AE) sensors for example, for detecting an ultrasonic wave accompanying an anomalous plasma discharge in a plasma processing equipment and for generating signals indicative of the ultrasonic wave detected (hereinafter referred to as ultrasonic signals;
data processing means for processing said ultrasonic signals; and
monitor means for displaying the signals.
The anomalous plasma discharge detection apparatus may comprise
a multiplicity of ultrasonic detection means, such as AE sensors, mounted on different components of a plasma processing equipment such as a vacuum chamber and electrodes for forming a plasma, for detecting the location of the anomalous plasma discharge in the vacuum chamber for example, the anomalous plasma discharge detection apparatus adapted to locate the source point by comparing on the same time axis the waveforms of an ultrasonic wave detected by these detection means to calculate delays in detection time (hereinafter referred to as delay times) by the respective ultrasonic detection means with respect to the first detection time by one of the detection means.
The anomalous plasma discharge detection apparatus may have only four ultrasonic detection means installed on the walls of a plasma processing chamber of the plasma processing equipment, such that the source point of the anomalous plasma discharge can be determined by an asymptotic approximation thereof through recursive calculations of the distances from the source point to the respective detection means in relation to the differences in delay time of the ultrasonic wave detected by the respective ultrasonic detection means.
The anomalous plasma discharge detection apparatus can be adapted to determine the level (or intensity) of an ultrasonic wave accompanying an anomalous plasma discharge, and also adapted to issue an alarm when the level is excessively high.
It is noted that the ultrasonic detection means may be provided with electrically insulated mounting means for mounting the ultrasonic detection means on the plasma processing equipment in an electrically insulated condition.
In another aspect of the invention, there is provided a method of detecting an anomalous plasma discharge in a plasma processing equipment by detecting an ultrasonic wave accompanying the anomalous plasma discharge.
In a still further aspect, the invention provides a method of locating the source point of an anomalous discharge in a plasma processing equipment by recursive calculations of the distances between the source point and four ultrasonic detection means based on the differences in delay time for the four ultrasonic detection means, the method using an asymptotic approximation of the source point through the recursive calculations thereof.
In a further aspect of the invention, the invention provides a dedicated hold case for accommodating and securing in position an acoustic emission (AE) sensor so that the AE sensor is in forced abutment against the lower case by an adequate pressure, thereby allowing an ultrasonic wave generated by an anomalous discharge to be transmitted to the AE sensor across their contacting surfaces. The hold case has a removable upper cover. With this apparatus, AE sensors can be glued on several positions of a processing chamber of a plasma processing equipment, especially on an existing apparatus, much easier than mounting them on the processing chamber without forming mounting bores. In addition, gluing the AE sensors on the processing chamber provides a better acoustic coupling between them. Further, when the sensors need to be removed from the apparatus for maintenance purposes for example, they can be easily removed by hitting them. Accordingly, the hitting may result in destruction of the AE sensors and deterioration of the internal structure, thereby failing to improve reproducibility of mounting the AE sensor.